Which engine do you have?
What size tires? (the larger the worse MPG)
How many miles are on your Suburban? (MPG generally will improve as you put more miles on)
How fast do you cruise on the highway? (a 5MPH difference can effect economy by as much as 2mpg).
The winter is not the best season to determine what kind of fuel economy your Suburban will get, winter blended fuels contain about 10% Ethanol along with extended warm up periods work against your average fuel economy and at highway speeds the thicker denser atmospher caused by colder air increases aerodynamic drag.
Wind can have one of the most dramatic effects on highway fuel economy, if you drive down a highway that runs North and South and you typically have a crosswind the blows from West to East this can adversely affect your fuel economy whether you are driving North or South. I drive a 750 miles stretch of I-15 regularly and it seams that (with my luck) no matter which direction I am headed Iím always fighting a headwind.
I get 18 on the highway with my '99 2wd. 4wd will never get as good as 2wd just because of the extra weight and component drag.
Synthetic oils in your transfercase and axles might help a little.
You may look into a cold air intake kit. They help on older rigs (I am not familiar with the '07's) and usually offer 1-2 mpg increase real-world. Tire pressure and tread design affect milage too. The biggest factor in vehicle milage also seems to be driving style, like ZX1100F1 stated too.
E85 won't net you better economy. Ethanol has less energy potential than gasoline, so it cannot provide the same energy output as gasoline. The upside of E85 is less emissions and as a renewable fuel source. Milage loss of up to 25% has been reported with E85 iirc. Do a google search on E85 for more info.
'99 Suburban 1500, 5.7 Vortec stock...for now.
'86 CJ7 daily driver, some mods, usualy dirty.
Most people buy an SUV of this size for family and safety!! I have an 02 3/4 tonwith the 6.0 motor The weight of this truck is 8600gross pounds(very heavy). I have 115,000kms (72000miles) and I get about 12mpg in the city and not to much better on the highway. I do have a heavy foot, but thats what a big motor is supposed to do get you going fast!!!. I did have a 93 2wd before this an mileage was definetly better. If you want better fule economy buy a Toyota Haha.
I had a Ford Explorer and it got only 15 to 16 MPG. Considering how much heavier the Sub is itreally isn't all that bad. I did find putting premium in the tank improved the MPG especially on the highway. The performance with premium was also definitly improved.
By me premium is $.20 to .30 more than regular. For the improved mpg (15%) and performance to me it was worth it. Just my opinion.
The best I have ever had is 16mpgs in my '07. That was shortly after I bought it. I have 23,000 miles on it & it isn't getting any better. I was getting 12 with gas city driving - we live in the country but I drive to town a lot. I now use E85 and get 10mpgs but it has been .80 per gallon less than gas and if I can help the farmers all the better.
I get 17-18 on the highway 13-14 highway/city combined in my 98 4X4. Hypertech makes a performance tuner for most vehicles. Anything you do to increase performance will increase economy in most cases if you can keep your foot out of it. For some vehicles, Hypertech also make an economy tuner that will tune your truck for better economy. This system will still provide your stock performance settings at full throttle so you have it when you need it. They don't make one for the 98 so I hve not had personal experience with this, but I did research it and when available for my truck, I will be purchasing one.